1. Field of the Invention
The present invention generally relates to the field of fiber production. More specifically, the invention relates to fibers of micron and sub-micron size diameters.
2. Description of the Relevant Art
Fibers having small diameters (e.g., micrometer (“micron”) to nanometer (“nano”)) are useful in a variety of fields from the clothing industry to military applications. For example, in the biomedical field, there is a strong interest in developing structures based on nanofibers that provide a scaffolding for tissue growth to effectively support living cells. In the textile field, there is a strong interest in nanofibers because the nanofibers have a high surface area per unit mass that provide light, but highly wear resistant, garments. As a class, carbon nanofibers are being used, for example, in reinforced composites, in heat management, and in reinforcement of elastomers. Many potential applications for small-diameter fibers are being developed as the ability to manufacture and control their chemical and physical properties improves.
Superhydrophobicity is an important property of a solid surface that can be demonstrated by a high water contact angle (“CA”)>150°. Superhydrophobic surfaces, along with having low contact angle hysteresis, usually less than 10°, will additionally have self-cleaning properties. These surface properties provide potential for several applications, such as windshields for automobiles, self-cleaning window glass, icephobic surfaces, textiles, construction, paints, microfluidics, Li-air batteries, and solar cells to name a few. Superhydrophobicity has been achieved by either decorating the surface with nano-micro size features of low surface energy materials or by attempting to develop nanofiber mats or membranes composed of low surface energy materials. Various processes have been conducted to produce these surfaces such as etching, lithography, mechanical stretching, layer-by-layer techniques, phase separation, electrochemical deposition, chemical vapor deposition, and electrospinning.
In the case of fiber formation of fluoropolymers (excellent candidates to prepare a superhydrophobic surface due to their extremely low surface energies), there have been various attempts to develop fiber mats. Fluoropolymers are classified as “non-melt processable” and, given their extremely low dielectric constant, these materials have been complex to process. A steady process for the preparation of ultrafine fluoropolymers (such as pure Teflon AF fiber) through electrospinning or any other technique does not appear to have been documented. Several attempts have pursued and are explained below.
A single step process to produce micro and nanofibers from polymer that has extremely high melt viscosity, such as PTFE, was proposed through a single step solvent-free technique. A mixture of PTFE (PTFE 601A or 7A powder from DuPont) and high pressure gas such as nitrogen and argon (up to 40%) was blown through a heated (260 C to 360 C) stainless steel nozzle. The outcome of the process was a small amount of pure PTFE fibrous material where a minimal amount of fibers with diameters as low as 30-40 nm and lengths as high as 3-4 mm are observed immersed within other structures. The degree of fibrillation was higher in jet blown fibers processed at temperature above the melting point of the material (e.g., 350 C for PTFE 601A or 7A powder). The water contact angle of the produced fibers was observed to be 147°.
Electrospinning is a well-known technique to produce micro and nanofibers. The first attempt to electrospin PTFE was done by electrospinning an aqueous dispersion of PTFE in distilled water with a nonionic wetting agent and stabilizer. The electrospun material was deposited on a conducting fluorine doped tin oxide coated glass slide fixed to a hot plate at 150 C. The process resulted in spraying the material instead of forming continuous fibers. The electrosprayed PTFE material was further heated to remove water and wetting agent. The as-sprayed PTFE coating was hydrophilic due to the presence of wetting agent, which then was removed by heating at 265 C in air or 190 C in vacuum. After this process, the coating showed a water contact angle as high as 167 with drop sliding angle of 2.
In another method of forming fluoropolymer fibers, a blend of a sacrificial matrix and the desired fluoropolymer are used in an electrospinning process to produce mixed fibers. The sacrificial matrix is then removed, leaving fibers the are primarily composed of the fluoropolymer. For example, an emulsion blend of poly(vinyl alcohol) (“PVA”) and PTFE in deionized water was electrospun. The PVA was removed by sintering the electrospun material resulting in fluoropolymer fibers. Electrospinning gave some fine fibers (300 nm) but mainly non-uniform fibers of PVA/PTFE at an emulsion mass ratio of 30:70. Increasing PVA content resulted in uniform fibers, but with a larger fiber diameter. DSC and ATR-FTIR studies before and after sintering confirmed complete removal of the sacrificial polymer, PVA, from the electrospun material. After sintering, however, the composite fibers were fused at the crossovers, forming a porous membrane. Generally fibers are not obtained as the ultimate material in such processes.
In another process, fluoropolymer fibers may be prepared by continuously coating an electrospinnable core polymeric material with the fluoropolymer. For example, a fluoropolymer (e.g., Teflon AF) may be continuously coated onto an electrospinnable polymer (e.g., poly(ε-caprolactone, “PCL”) that is used as a core material to form superhydrophobic coaxial fibers. Teflon AF 2400, 1 wt % in Fluorinert FC-75 solvent (400-S1-100-1 purchased from DuPont) was used as sheath material and PCL dissolved in 2,2,2-trifluoroethanol (TFE, 99.8% purity) solution was used as core material. Uniform coaxial fibers were formed with 10 wt % PCL core at 1.5 mL/hr and 1 wt % Teflon sheath at 1 ml/hr feed rate with diameter within ˜1-2 μm. The coaxial PCL/Teflon AF fiber exhibited water contact angle of 158 and contact angle with dodecane of around 130 while the same value for PCL only fiber were 125 and 0, respectively. Other materials (e.g., poly(vinyledenefluoride) or poly(acrylonitrile)) may be used as a core material.
There are many problems with coaxial electrospinning methods. Coaxial electrospinning methods generally exhibit a lack of uniformity in the fibers observed. Additionally, the core does not always stay as the core and the sheath is generally not a uniform coating of the core material. Additionally, the process of forming coaxial fibers is complex and uses significant amounts of solvents. While some few attempts have been made to prepare pure fluoropolymer fibers, no attempts appear to be readily scaled up for commercial use given the above noted deficiencies.